1. Field of the Invention
The field of this invention is that of epoxy molding compositions suitable as encapsulants for electrical and electronic devices, particularly for microelectronic components such as semiconductors.
2. Description of the Prior Art
Electrical and electronic devices have been encapsulated in a variety of resinous materials, including epoxy, silicone and phenolic materials. Thus, it is known to provide epoxy resinous molding compositions which comprise epoxy resin, hardener therefor and inorganic filler as the essential ingredients thereof, to which various adjuvants such as catalysts, mold release agents, pigments, flame retardants and coupling agents may be added.
The following performance characteristics are required of resinous encapsulants for microelectronic devices:
a. good device compatability, with no chemical, physical and electrical interference of performance of semiconductor devices by the plastic encapsulant used; PA1 b. adequate sealing of leads to prevent penetration of moisture and ionic contaminants along leads; PA1 c. low moisture penetration through the encapsulant; PA1 d. low level of ionic contaminants such as Li.sup.+, Na.sup.+, K.sup.+, and Cl.sup.- ; PA1 e. High glass transition temperature; PA1 f. Low coefficient of thermal expansion; PA1 g. High thermal conductivity; PA1 h. Long term dimensional stability. PA1 A-162--Methyl triethyoxy silane PA1 A-163--Methyl trimethoxy silane PA1 A-172--Vinyl-tris-(2-methoxyethoxy) silane PA1 A-186--Beta-[3,4-epoxy-cyclohexyl] ethyl trimethoxy silane PA1 A-187--Gamma-glycidoxypropyl trimethoxy silane PA1 A-1100--Gamma-aminopropyltriethoxy silane PA1 KBM-202--Diphenyl dimethoxy silane--available from SHINETSU CHEMICAL CO., and
While the requirements listed in a, b, c, d, e and h are largely fulfilled by epoxy resins cured or hardened with anhydrides, phenol formaldehyde condensates, cresol formaldehyde condensates, polyamines or combinations thereof, with or without catalysts and coupling agents, the crucial factors of low coefficient of thermal expansion and high thermal conductivity, together with the required low level of ionic contaminants and minimal abrasiveness are influenced directly and significantly by the choice of the inorganic filler or fillers.
The importance of a low coefficient of thermal expansion in an epoxy molding composition cannot be overemphasized. The tremendous progress in the microelectronics industry has enabled the production of semiconductor chips of increasing size, functionality, complexity and circuit density. Such large semiconductor chips are more vulnerable to thermally-induced stress than the small, simpler chips, with the result that the use of an encapsulant composition which does not have a low coefficient of thermal expansion causes premature failure due to cracking of chips, wire breakage, cracking of passivation layer and parametric shift. Such defects are all related to a large thermally induced internal stress, the result of a high, rather than a low, coefficient of thermal expansion of the epoxy molding composition employed.
In addition to the requirement of a low coefficient of thermal expansion, it is equally important that the epoxy molding composition for encapsulating semiconductor devices have a high thermal conductivity. Semiconductor devices of high circuit density generate more heat per unit area than devices of low circuit density, requiring the rapid dissipation of heat through the encapsulant in order to insure cool operation and a long operating life. It is widely accepted in the electronics industry that an increase of 10.degree. C. in junction temperature decreases the life expectancy of a semiconductor device by one half. Therefore, the property of high thermal conductivity, i.e., rapid dissipation of heat, is necessary to the efficient operation and long life of a microelectronic device.
The inorganic fillers currently in use for epoxy molding compositions include fused silica, .alpha.-quartz, alumina, fiber glass, calcium silicate, a variety of earths and clays, and various combinations thereof. These fillers, which are present to the extent of from about 40 to about 80 percent by weight of the total epoxy composition, exert the greatest influence on the properties of coefficient of thermal expansion and thermal conductivity. Thus, fused silica exhibits a low coefficient of thermal expansion. Unfortunately, this same filler also has a low thermal conductivity. It must therefore be used in conjunction with a filler having a high thermal conductivity to provide these dual properties.
Another widely used filler is .alpha.-quartz, which exhibits a high thermal conductivity, but also a high coefficient of thermal expansion, requiring that it be used in combination with a filler of low coefficient of thermal expansion to overcome this deficiency.
Although alumina does exhibit the dual properties of a low coefficient of thermal expansion and a high thermal conductivity, its excessive abrasiveness precludes its use as it causes an unacceptably excessive and rapid wear of manufacturing and molding equipment.
The prior art fails to disclose a practical, useable filler which possesses a low coefficient of thermal expansion, which is here defined to be less than 23.times.10.sup.-6 /.degree.C. below the glass transition temperature, and at the same time a high thermal conductivity, here defined as one greater than 25.times.10.sup.-4 cal./.degree.C. per centimeter per second. The procedures for determining the coefficient of thermal expansion and the thermal conductivity are described hereinafter in Example 1.
Accordingly, it would be desirable to provide an improved epoxy molding composition containing a filler which exhibits the dual properties of a low coefficient of thermal expansion and a high thermal conductivity and which filler is relatively non-abrasive and free of ionic contaminants.
It is therefore the principal object of this invention to provide an improved epoxy molding composition which contains a filler which contributes to the composition the dual properties of a low coefficient of thermal expansion and a high thermal conductivity and which is relatively non-abrasive and free of ionic contaminants.